For about one year, I have noticed my testes are larger than normal.

A doctor(on a unrelated matter) noticed as well, and palpated
the testes, and did not notice any abnormalities or lumps.

When its warm, they descend normally, and are soft. When its
cool they retract normally, and are larger than normal.

What could possibly cause this? I am a bit past 50, and in
excellent health (I exercise vigorously every day, for an hour or more).

What could possibly cause this? Increased testosterone? Mercury from fish? Hydrocele? Aging and expansion of soft tissue?